Continuous electrolytic cell process



March 31, 1970 a. J. cams CONTINUOUS ELECTROLYTIC CELL PROCESS FiledApril 5, 1965 2 Sheets-Sheet 1 FIG. I

United States Patent Int. Cl. C01b 11 /18; B01k 1/00 US. Cl. 204-82 3Claims ABSTRACT OF THE DISCLOSURE The operation of multipolarelectrolytic cells in the production of an alkali metal chlorate orperchlorate, particularly sodium chlorate or perchlorate, by theelectrolysis of an electrolyte comprising an aqueous solution of analkali metal chloride on a continuous basis. In the continuousoperation, the electrolyte is continuously recirculated through the cellfrom a cell tank to effect electrolysis, a small proportion of therecirculating volume of the electrolyte being continuously withdrawnfrom the cell tank and fresh electrolyte being continuously fed to thecell tank. By the improved operation of the multipolar cell according tothe present invention, the overall efficiency of the process for theproduction of the alkali metal chlorate or perchlorate is substantiallyincreased.

This invention relates to a new and improved process and apparatus forthe electrolysis of brine and more particularly for the continuouselectrolysis of an alkali halide brine to alkali metal chlorates andperchlorates carried out in multipolar electrolytic cells.

The most economic manufacture of sodium chlorate and perchlorate isusually performed in multipolar electrolytic cells generally installedin a large tank. These cells comprises a series of parallel electrodesmounted in a box and sealed at their edges and bottom to prevent leakageof solution or electric current, the solution to be electroylsed beingmaintained between each set of electrodes. A brine such as NaCl iselectrolysed in the cells through a number of intermediate reactions toform the sodium chlorate, with hydrogen gas evolved as a by-product whenoxygen from the water is freed to enter into the reaction. The overallchemical reaction is represented by:

The brine solution containing NaCl and some NaClO circulates betweenthe'electrodes and at each pass some of the NaCl is converted to NaClOFresh solution enters each electrode compartment at the lower edge ofthe electrode through an inlet pipe, and the natural circulation of theliquor between the electrodes is caused by the generation of hydrogengas between the cell plates. This reduces the average specific gravityof the material between the electrodes. The heavier dense fresh liquorfrom the tank enters into the bottom inlet pipes and forces out themixture of liquid and gas through upper pipes positioned at the top ofthe electrodes. In addition, a minor circulation force is caused by atemperature increase of the electrolyte in the cells between theelectrodes. A number of these large tanks will be located in a chlorateplant and all of these tanks will be operated on a batch principal. Theusual output from a single tank varies from between 2000 and 2000 tonsper year depending on its size.

Operating a large tank as a batch process has many disadvantages. Forexample, the electrolyte in each tank must be batch operated andseparately analysed and controlled by adding various materials such aswater, brine, acid, chromate, a soluble calcium, salt, etc., when theybe- 3,503,858 Patented Mar. 31, 1970 come depleted in the electrolyte.Furthermore, as the electrolyte becomes enriched with the desiredcomponents, the voltage necessary for maintaining the current flowthrough the cell is increased. As an example, a freshly filled tankwould require only volts for a full production rate, but the same tankwhen the electrolysing is finishing would require about volts. Sincethis variation takes place across each cell tank, the voltage supplymust be variable over a relatively large range. This increases thecapital cost of equipment and increases the peak demand for electricalenergy; hence the cost per kilowatt hour of electric power willincrease.

In addition more electric power is required for a greater finalconcentration of NaClO In the usual plant a final concentration ofbetween 450 g.p.l. and 9 00 g.p.l. of NaClO is necessary so that theNaClO crystals can be economically extracted from their mother liquor.

Large storage containers are required if a tank is to be quickly filledafter it has been emptied, and in addition other large storagecontainers are required to receive the electrolysed product when it isremoved from a cell.

During the emptying of a cell tank after completion of a process run,the entire facility on the same circuit must be disconnected. Sinceemptying and filling a tank required approximately 1 to 3 hours, atypical plant of 4 to 8 cell tanks on one circuit will incur a lostproduction time offrom 5% to 15%.

The use of batch operations further requires the need of a high capacitypump to quickly fill and empty these tanks. Also operating time is loston commencing a new electrolysis run since the temperature must beallowed to come to equilibrium operating conditions. Finally the suddentemperature changes which are produced on the tank/cell box willadversely affect the component parts after sufficient use.

It is an object of this invention to overcome the drawbacks of the priorart and to provide an increase in productivity in the order of 5%.

Further objects of the invention will become apparent from thedescription to follow.

The objects of the invention are attained in an apparatus comprising amultipolar cell contained in a cell tank, means for supplying fresh feedto said cell tank, means for internally recirculating electrolysedliquor within said multipolar cell and means for withdrawing enrichedelectrolysed liquor from said multipolar cell.

In the drawings:

FIG. 1 represents a perspective View of a multipolar electrolytic cell;and

FIG. 2 represents a perspective view of the cell embodying therecirculation system according to this invention.

In the diagrams, an electrolytic multipolar cell tank is shown havingside walls 1, monopolar electrodes 2 and multipolar electrodes 3 ofgraphite, or other conducting material. The multipolar cell is containedin a cell tank 5 containing an alkali earth' metal brine such as sodiumchloride, the multipolar cell being immersed to level 7. Since theelectrolyte in the multipolar cell is at a level 6, the hydrostaticdifference in the two levels allows the electrolyte to flow out throughupper outlet tubes 10 while liquor from cell tank 5 enters themultipolar cell through inlet tubes 4.

According to this invention, the outflow through tubes 10 is internallyrecirculated from the electrolytic cell 1 to the cell tank 5. This mayconveniently be done by passing the outflow through the open ends ofoutflow troughs 8, then through the larger body of liquor in the celltank 5 where it is cooled by coming. in contact with cooling coils (notshown) and back through the inlet tubes 4. This will be continued untila suitable concentration of NaCl and NaClO is produced. As shown in FIG.

2, pump P removesla very small proportionofliquor from the collectingtrough or if desired (but not shown) from cell tank 5 into storage orfor further processing, etc. Fresh NaCl brine is added to the mainliquor body of the tank through inlet tubes 9 to adjust the liquor tothe desired concentration of NaCl and NaClO When the desiredconcentration is reached some of the liquor is pumped from the outflowtrough to processing equipment where hypochlorites, chromates, graphiteor other deleterious matter is removed. Nothing in this invention limitsthe feed of impure solution containing all or a part of these impuritiesdirectly into the processing equipment. Chromates contaminants can beadded for instance to a chlorine dioxide generator without harm. A finaladjustment to the liquor requiring addition of a small amount of NaCl asbrine or crystals or of NaClO crystal may be required for preciseadjustment of the product. According to this invention only a part ofthe liquid is drawn 011 through outflow trough 8. Fresh treated brineenters the bottom of the tank 1 compensating in volume for that leavingthrough the outflow trough. Any desired concentration of NaCl or NaClOcan be obtained direct ly from the cell tank as within the followinglimits:

(a) a product containing NaCl at saturation and a trace of NaClO (b) aproduct containing 40 g.p.l. NaCl and 900 g.p.l.

NaClO In particular the specific feed for chlorine dioxide generatorscan be made'most economically.

In the present invention the final concentration of NaClO is in mostcases only about 300 g.p.l. NaClO versus a final concentration of450-900 g.p.l. for the usual processes. Consequently the electric powersaving in the present invention for direct feed to a chlorine dioxidegenerator will be between 3 and 8%.

I claim: I

1. In the production of a compound selected from an alkali metalchlorate and an alkali metal perchlorate which includes electrolysing anelectrolyte comprising an aqueous solution of an alkali metal chloridein a multipolar electrolytic cell comprising a plurality of spacedbipolar electrode plates forming a plurality of separate individual cellspaces, opposite surfaces of each bipolar electrode being of oppositepolarity in respect of the electrolyte in which they are immersed, thecurrent flow being normal to said electrolysing surfaces and theelectric potential in any one of the spaces between the electrodes beingdifferent from that in the next adjacent space across the cell, in whichprocess the electrolyte is continuously recirculated upwardly throughsaid cell during electrolysis from a cell tank surrounding said cell toeffect miXing of the electrolyzed electrolyte from each of said cellspaces only in said cell tank for equalization of electric potential andcompletion of reaction of the electrolysis products to produce saidchlorate or perchlorate, the improvement which comprises continuouslywithdrawing a small proportion per hourof the volume of electrolyterecirculating in said cell tank and continuously introducing freshelectrolyte into said cell tank whereby to effect the process on acontinuous basis.

2. The process as claimed in claim 2 in which the alkali metal issodium.

3. The process as claimed in claim 1 in which said small proportion isup to 10% by volume per hour of the recirculating electrolyte in a celltank.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner D.R. I ORDAN, Assistant Examiner US. Cl. X.R.

